Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
U.S. Pat. No. 6,085,528 (the “PLNG Patent”), having corresponding International Publication Number WO 98/59085 and entitled “System for Processing, Storing, and Transporting Liquefied Natural Gas”, describes containers and transportation vessels for storage and marine transportation of pressurized liquefied natural gas (“PLNG”) at a pressure in the broad range of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at a temperature in the broad range of about −123° C. (−190° F.) to about −62° C. (−80° F.). Containers described in the PLNG Patent are constructed from ultra-high strength, low alloy steels containing less than 9 wt % nickel (“Steel PLNG Containers”). The PLNG Patent is hereby incorporated herein by reference. As used herein, “ultra-high strength, low alloy steel” means any steel containing iron and less than about 10 wt % total alloy additives and having a tensile strength greater than 830 MPa (120 ksi).
Steel PLNG Containers, as well as other metallic containers for storing pressurized, cryogenic temperature fluids, typically include welded joints. The weld joints must have sufficient resistance to fracture initiation, i.e., toughness, since they may contain discontinuities that can affect the mechanical integrity of a metallic container. See Glossary for definition of weld joint. Typical weld discontinuities include, for example, lack of penetration, lack of fusion, hydrogen cracking, and inclusions. Welding operations can degrade toughness by degrading the metallurgy in what is referred to as the heat-affected-zone (“HAZ”), which is the base metal that is adjacent to the weld fusion line and that was affected by the heat of welding. For certain applications where HAZ toughness is a design limiting issue, common methods of improving and controlling HAZ toughness are to limit welding heat input to low values or to use welding techniques that better control heat input, such as gas tungsten arc welding (“GTAW”) instead of submerged arc welding (“SAW”). Unfortunately, these methods are costly in that the welding operations are much more time consuming, expensive equipment and consumables are needed, special training is required for welders, and/or quality control and assurance methods are onerous.
It is desirable to have economically acceptable methods for commercial welding of metallic containers for storing pressurized, cryogenic temperature fluids that provide weld joints with appropriate strength and toughness.